In 1988, matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) was introduced by Hillenkamp and Karas (Anal. Chem. 60:2288-2301, 1988), and, has since become a valuable tool for protein characterization and identification. Briefly, MALDI-TOF mass spectrometry is based on the ability to generate intact vapor-phase ions of large, thermally labile biomolecules by desorption/ionization from a matrix comprised of small volatile (matrix) molecules and the biomolecules or other large molecules to be studied. Pulsed laser radiation, that is absorbed by the matrix is used to initiate the desorption/ionization event and to simultaneously generate a packet of ions of having different mass-to-charge ratios (m/z). These ions are accelerated to the same electrostatic potential and allowed to drift an equal distance before striking a detector. The mass of the ions is determined by the flight times of the ions.
However, even with MALDI-TOF, there are difficulties in performing mass spectrometry in the high mass range (>30 kDa), and even more obstacles in performing mass spectrometry in the ultra high mass range (>100 kDa). There are three fundamental problems associated with mass spectrometry of high mass species. The first problem involves removal of the enormous amount of kinetic energy imparted to the high mass species in moving them from atmospheric pressure or a condensed matrix into vacuum during the ionization/vaporization process. The second problem is that most mass analyzers are not designed or are physically incapable of working in the high or ultra high mass range. Thirdly, there are challenges with detecting the analytes as a function of increasing mass-to-charge ratio. As known in the art, detection efficiency decreases significantly with increasing mass above approximately 104 Da.
Consequently, large molecules such as proteins have to be fragmented or multiply charged so that they can be analyzed in the working range of the mass spectrometer. This makes quantitation and characterization of large molecules, such as proteins, very difficult and time consuming.
MALDI has been combined with an ion trap mass spectrometer, and MALDI has been practiced as aerosol MALDI. A paper by the two present inventors, Harris et al., entitled “Aerosol MALDI of peptides and proteins in an ion trap mass spectrometer”, International Journal of Mass Spectrometry 258 (2006) 113-119, discloses utilizing the aerosol MALDI technique with a digital ion trap mass spectrometer. In a digital ion trap (DIT), quadrupole trapping and excitation waveforms are generated by rapid switching between discrete d.c. voltage levels. As the timing of the switch can be controlled precisely by available digital circuitry, this approach provides an opportunity to generate mass spectra using a frequency scan, in contrast to the conventional voltage scan used by conventional ion traps, thus providing a wider mass range of analysis. Such an arrangement is disclosed to be advantageous because the resolution and signal-to-noise ratio are not products of the laser ablation event. Trapping and detection of ions up to m/z 16.9 kDa (myoglobin) are disclosed in Harris et al. This Paper highlights the need for a method for injecting ions into the trap so that the working range of the spectrometer is not limited, such as to enable measurement of large molecules having a mass >20 kDa, but does not disclose or suggest a system for solving this need. Accordingly, there is a need for a new method for injecting ions into a DIT to extend the working mass range of a DIT-mass spectrometer, such as to enable mass spectrometry to permit real-time analysis of large molecules having a mass of over 20 kDa, such as proteins, viruses, whole DNA and RNA, whole bacteria, pollen and other ultra high mass species.